The present invention relates to an anode for a hooded, fused salt electrolytic cell for producing aluminum, said anode having an anode rod and at least one carbon block attached to an anode rod pin.
In the production of aluminum by the fused salt electrolysis of aluminum oxide, the aluminumoxide is dissolved in a fluoride melt made up for the most part of cryolite. The cathodically precipitated aluminum collects under the fluoride melt on the carbon floor of the cell, the surface of the liquid aluminum itself forming the cathode. Dipping into the melt from above are anodes which in conventional processes are made of amorphous carbon. At these carbon anodes oxygen forms as a result of the electrolytic decomposition of the aluminum oxide. The oxygen reacts with the carbon of the anodes to form CO.sub.2 and CO. The electrolytic process takes place in a temperature range of around 940.degree.-970.degree. C.
During the course of the electrolytic process the electrolyte becomes depleted of aluminum oxide. At a lower aluminum oxide concentration of 1-2% in the electrolyte the anode effect occurs whereby the voltage rises from 4-5 V to 30 V and more. At this time the concentration of aluminum oxide must be raised by the addition of alumina to the cell.
The present day environmental requirements require the waste gases produced during the manufacture of aluminum to be collected and passed through a suitable waste gas treatment unit before release to the atmosphere.
Numerous systems for covering the cell on all sides have been proposed in order to meet these requirements. The waste gases are led off via a pipe running the length of the cell along its central axis.
Known hooding systems are made up essentially of covers positioned horizontally below the anode beam around the anode rods (DE-OS 2 330 557) or the anode blocks (DE-OS 2 251 898) with other side covers attached at the sides and sloping down to the edge of the cell. The sloping covering at the side is subdivided into a series of hoods. When attending to the cell one or more hoods or even the whole sloping side covering can be raised by means of a supporting frame.
According to a further version described in DE-OS 2 510 400 the hooding is attached to the anode beam itself and can be tilted up by a two-stage tilting movement around an axis of rotation laying in the longitudinal direction of the cell.
Finally, according to the published pat. appl. DE-OS 2 263 348 the parts of the hooding forming the covers at the longitudinal side of the cell can be pushed inside one another.
In spite of their relatively expensive construction, these known hooding systems suffer from a number short-comings. In particular, in DE-OS 2 263 348, when changing anodes, either an extensive part of the cell must be uncovered or else the anodes cannot be readily changed. Furthermore, raising the anode beam, for which an auxiliary traverse is usually employed, is not always possible without some difficulty.
A further disadvantage of these known hooding systems lies in the economics of these systems. The financial outlay is relatively large and retrofitting non-hooded cells, if at all possible, involves very great expense.
In U.S. Pat. No. 4,218,300 a more economical hooding is disclosed which is also more suitable for attending to the cell. The hooding comprises very light, throwaway hoods which can slide both on the anode beam and on the edge of the cell.
Stops limiting the raising movement ensure that in the upper range of the raising movement the lightweight hoods slide on the edge of the cell, in the lower range on the other hand only on its vertical part.
A disadvantage which has to be accepted, however, with such lightweight hoods is that they are often bent or otherwise deformed and therefore no longer air-tight. To maintain a properly functioning covering for the cell a considerable amount of checking of these parts is therefore necessary. One must decide whether and when the defective hoods have to be replaced.
It is therefore the principal object of the present invention to develop a hooding system for electrolytic cells employing throwaway hoods which are easy to change and do not require any special work of inspection.